1. Field of the Invention
The present invention generally relates to a radio frequency receiver, and more particularly, to a burst detection apparatus and method for radio frequency receivers.
2. Description of the Prior Art
In a communication field environment, the reception methods of RF (radio frequency) signal have to distinguish the desired signal from the noise. Naturally, the desired signal and noise cannot be fully recognized by these reception methods. Therefore, the probability of mistaking noise as signal is denoted as Psignal|noise. Like wise, the probability of mistaking signal as noise is denoted as Pnoise|signal. It is desired in the field of the art to reduce both the above-mentioned probabilities.
A so-called “burst detection” method is used to determine whether the received signal is a desired signal or noise. In some communication systems, wrong outcomes of burst detection method would lead to communication delays, interruptions, data drops, or even channel breakdowns. For example, the well-known GSM (Global System for Mobile communications) specification 11.10 defines that the frequency emitted should be kept within a precise range. Usually, an AFC (Automatic Frequency Control) mechanism is provided to maintain the precision of frequency. Besides, the GSM specification 11.10 requires that during a temporary gap of absence of the required signal, the MS (Mobile Station) shall maintain the frequency and timing of its transmission. If the received waves is determined by the burst detection method as noise, the frequency estimated by the AFC will be cast away in order to maintain the precision of frequency. If the result from the frequency estimation of noise is accepted, the consequence may lead to the loss of traffic or even a release of a channel.
A well known burst detection method, the correlation algorithm, is chosen to determine a correlation value. In order to perform this correlation method, a pre-acknowledged training sequence is included in the signal burst as well as the synchronization should be performed. Since noise has little correlation with the acknowledged training sequence, whenever the strengths of the received signals are equal, the correlation value of noise generated by the correlation algorithm should be much less than the correlation value of the desired signal. Hence, a threshold value is compared with the normalized correlation peak value in order to determine whether the received signal is a desired signal or noise. In a GSM system, a correlation value below the given threshold value leads to a rejection of the frequency estimated by the AFC function. Otherwise, a correlation value higher than the chosen threshold guarantees that the estimated frequency by the AFC would be adopted.
By adjusting the selectable threshold value, Psignal|noise can be reduced to an acceptable level effectively while keeping Pnoise|signal at a fairly low level. However, the method has two shortcomings. First, the length of training code sequence is merely 26, false correlation peak value may be appeared. It is possible to mistake noise as signal. Second, in multi-path propagation, if signal strengths of two or more paths are similar, the normalized correlation peak value according to the strength of one path may be lower than the threshold. In consequence, it is possible to mistake signal as noise.
In summary, it is desired to have a receiver apparatus and method to reduce the probabilities of mistaking noise as signal, and vice versa.